Table of Contents

Mechanics

Menu

Prev

Next

Volume, Density and Specific Gravity
As the pressure on a fluid changes its volume will also change. For a gas, the change in volume may be large whereas for most liquids it will be small. The gas is said to be a compressible material and the liquid an incompressible material. Volume, V, is a parameter that depends on the size of the system, and in the case of the earth's atmosphere can be very large. It is, therefore, frequently convenient to identify the volume per unit mass of a material as its "specific volume:"
v = (V/m). The specific volume is the reciprocal of the density of the material, which is the mass per unit volume: r = (1/v) = (m/V).

The "Specific Gravity" of a substance is the ratio of its mass to that of an equal volume of water at the same temperature and pressure. This is also the ratio of the densities of the two substances:
SG = (mass of a volume V of a material)/(mass of a volume V of water) = (rMaterial/rWater).

Two fluids that will be of interest in human-powered transportation are water and air. In the metric system a volume of water was used to define the unit of mass. At 4 C water has its maximum density of 1000 kg/m3. As the temperature is increased the thermal expansion of the liquid increases the volume of this mass of material and the density decreases. At 20 C (room temperature) the density is 998.2 kg/m3.

Air at atmospheric pressure (1.013 x 105 Pa.) and 20 C has a density of 1.204 kg/m3. The specific Gravity of air is the ratio of the densities of air and water:

SG(Air) = (rAir/rWater) = (1.024/998.2) = 1.03 x 10-3.